She is a priestess that knows about the Treasures' secrets.
She is recruitable when the player obtains the Magic Magic from Kai.
Kai is initially an impressive character, 25 Magic, and an Water Magic level of 35, this makes Kai the best Water mage ypu can get in the whole game. If you can, teach to her the Hidden Spell Art Tempest, this shall reap giant damage.
She also can use Rapiers, Daggers, Bows and Guns quite nicely because she has 19 in both Dexterity and Speed.
Her cosplay item, the Goddess Statue is obtained pretty late. But it turns her into Goddess, she gets 12 points in Strength, 3 in Dexterity and 6 in Stamina, plus all the the benefits from the Goddess' Statue.
Kai is useful, but the Goddess cosplay becomes avaliable late when the last boss remaining is Saiva, when you have grown characters like Narcisse, Blue/Rouge, Volcano, Orion, Kremhild, Zozma, Hector... at your disposal.